[PullPreview] shutdown pullpreview instances when inactive

[mszulik]

We don't want to have pullpreview instances run forever, if we forgot to remove the label.

We can use the ttl parameter provided by the pullpreview workflow for this:

https://github.com/pullpreview/action/blob/2edc1ba5ed473d0723cbf94680a93b1c0ebb9f5b/.github/workflows/pullpreview.yml#L39